1. Field of the Invention
This invention relates to an ionization smoke detector having a characteristic moisture-proof structure for preventing penetration of moisture etc. into a circuitry-accomodating portion and having a characteristic structure for mounting electrodes on an insulating board within a detector cover.
2. Prior Art
As conventional ionization smoke detectors, there may be mentioned, for example, a smoke detector as illustrated in FIG. 16.
In FIG. 16, 50 is a detector body, 51 is a body cover and 52 is an outer cover. The outer cover 52 has smoke inlets 53.
An insulating board 54 is disposed in the detector body 50 comprising the body cover 51 and the outer cover 52. An inner electrode 56 with a radiation source 55, an intermediate electrode 58 with a transmitting aperture 57 and an outer electrode 59 into which external smoke may enter are supported and fixed on a foreside of the insulating board 54. These electrodes constitutes an electrode arrangement for ionization smoke detection. A printed circuit board 60 with detector circuitry packaged thereon is placed on a backside of the insulating board 54.
A space for accommodating the printed circuit board 60 is sealed, at its lower portion, with the insulating board 54 through a rubber packing 61 and, at its upper portion, with an upper lid 63 through a rubber packing 62, to prevent penetration of moisture or corrosive gases.
However, the number of parts to be employed is large and the manufacturing process is complicated in the conventional ionization smoke detector as described above, because the rubber packings 61, 62 are used for moisture preventing structure of the circuitry accommodating portion. In addition, the rubber packings 61, 62 increase a height of the detector, which makes it difficult to reduce the size of the detector.
In the conventional ionization smoke detector, an intermediate electrode 58 is mounted by screws on the insulating board 54 through a spacer 64 for forming an inner ionization chamber between the intermediate electrode 58 and the inner electrode 56.
In such an arrangement, a space is needed to fix the intermediate electrode 58 to the insulating board 54 by screws. This increases the space for mounting the intermediate electrode 58 and makes the assembling operation complicated because it needs screw fastening operation.
Furthermore, the conventional ionization smoke detector has such a disadvantage that an electrode lead of the intermediate electrode 58 extends through the insulating board 54 to be connected to a lead of an FET on the backside of the insulating board 54. This allows moisture or corrosive gases to enter the circuitry-accommodating space through the opening of the board through which the lead extends. This will possibly corrode the circuit parts.